Variable Speed Audio Playback Using Enhanced Correlation
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Solution Overview
Problem
Existing variable speed playback techniques for digital audio signals often introduce pitch distortion and artifacts, such as a 'chipmunk-like' effect when speeding up or slowing down audio, and produce noticeable reverberations and clicks due to discontinuities in the signal.
Innovation Solution
The proposed variable speed playback system uses a framework similar to Synchronized Overlap Add (SOLA) but with improvements including a 20 ms frame length, overlapping regions of 15 ms, and a novel enhanced correlation technique that weights local maxima by their distance from the center, allowing for a higher input-to-output ratio and better fidelity, especially in music with dynamic pitches.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the audio signal is played back at a different sampling rate to achieve variable speed playback, then the playback speed is improved, but the audio pitch is distorted
Solution Approach 1:
The audio signal is divided into short frames of fixed length (e.g., 100 ms). By segmenting the continuous audio signal into discrete frames, the system can manipulate individual frames independently to achieve speed variation while maintaining pitch integrity through cross-fade transitions between frames.
Solution Approach 2:
The system applies periodic cross-fade transitions between consecutive audio frames at a rate matching the frame duration. This periodic overlapping and fading creates smooth transitions that eliminate discontinuities and artifacts while maintaining the desired playback speed and pitch relationship.
2Speed
If short audio frames are discarded and connected to achieve pitch-invariant variable speed playback, then the playback speed is improved, but artifacts and signal distortion are introduced
Solution Approach 1:
The system pre-calculates and applies cross-fade transitions between frames before the actual playback occurs. By preparing the smoothed transitions in advance during frame connection, the system eliminates abrupt discontinuities that would otherwise cause audible clicks and artifacts in the output signal.
Solution Approach 2:
The cross-fade transition acts as an intermediary element between consecutive audio frames. This intermediate smoothing function gradually blends the end of one frame with the beginning of the next, preventing direct hard connections that cause discontinuities and improving overall signal quality.
3Reliability
If the OLA technique is used to eliminate clicks by overlapping frames, then click artifacts are reduced, but reverberations are introduced
Solution Approach 1:
The system applies a partial overlap (cross-fade) between frames rather than a complete overlap. By using a controlled overlap duration that is less than the full frame length, the system achieves sufficient smoothness to eliminate clicks while avoiding excessive overlap that would cause reverberation artifacts.
Data Source
AI summary
A method and system for modifying a digital audio signal to vary its playback speed while preserving the signal's pitch and quality. The variable speed playback (VSP) system and method mitigates artifacts remaining after processing by existing techniques. The VSP system and method produces a consistent and pleasing sound to an audio file, even while its speed is varied during playback. The VSP method includes selecting and estimating an input frame, adjusting the frame position, and overlapping and adding the adjust frame to an output signal. The frame position adjustment is achieved using an enhanced correlation technique that finds all local maxima over a cross-correlation function. The local maxima having a highest correlation score is designated as a cut position, where the adjusted frame is cut from the input buffer. The VSP system and method using four input frames to generate one output frame.